The invention relates to papermaking machinery and methods. The invention relates more particularly to measuring arrangements for measuring properties of a moving paper web in a shortened dry end of a tissue machine.
In the production of paper, it is common practice to monitor the condition of the paper web at certain points along the web""s passage through the papermaking machine, and to adjust the papermaking process as necessary depending on the condition of the web. For instance, in the dry end of a paper machine, the web exiting from the final drying section of the machine is typically monitored to measure properties such as the basis weight and moisture. In particular, the profiles of such properties in the machine direction (MD) and cross-machine direction (CD) are deduced from the measurements. An ideal web would have flat MD and CD profiles of all parameters of interest. In the real world, such profiles are never flat at all times. Through monitoring of the profiles and appropriate adjustment of the papermaking process in a closed-loop control, however, the profiles can be maintained close to the desired flat state.
Various types of sensor devices have been used for monitoring the properties of a moving paper web. The sensor devices typically comprise optical sensors employing light transmission through the web and/or reflection from the web, and detection of the transmitted or reflected light, from which paper properties of interest are deduced. With most types of sensors, it has been the conventional practice to mount the sensors on a measuring frame whose only or primary purpose is to support the sensors. For example, in a conventional tissue machine employing a final Yankee dryer, a measuring frame located just downstream of the Yankee dryer and upstream of the reel-up supports sensors for measuring basis weight and moisture content of the finally dried web.
In some paper machines, the sensors are traversed in the cross-machine direction. The direction of traverse is normally substantially perpendicular to the direction of movement of the web. The sensors therefore measure properties of diagonal samples of the web, rather than the whole web. Measurements are made at substantially the same plurality of locations across the machine during each traverse, and they may be made while traversing the sensors in one or both directions across the web. Measured variations in web properties of interest are commonly separated by means of numerical algorithms into estimates of the MD and CD variations. The usual separation methods attempt to identify MD variations and to separate them from the scan data, and the remaining variations are considered to be CD and random variations. MD variations of high frequency cannot be separated and are commonly deemed to be random variations. Variations designated as random are often removed by filtering. MD variations of low frequency may be substantially identified and separated with any of several numerical algorithms. Such algorithms include averaging, exponential filtering, or Kalman filtering applied to each cell.
A drawback of the conventional arrangement is that the measuring frame takes up space in the machine direction and, consequently, the draw between the Yankee dryer or other final dryer and the reel-up becomes somewhat long. As a further consequence of this long draw, the paper web must be supported between the final dryer and reel-up, or else the web will not be stable and will not be capable of supporting its own weight without risk of breaking. Thus, sophisticated supporting equipment is required.
Traversing sensors also tend to collect fibers and debris and hence must be regularly cleaned to maintain their proper operation and to prevent dust accumulation that can present a fire hazard.
Still another disadvantage of typical measuring arrangements is that basis weight is measured by placing a source of radioactive isotopes on one side of the web and a detector on the other side of the web. The detector receives the radioactive rays after their passage through the web and deduces basis weight based on the degree to which the web absorbs the radiation. Not only are radioactive emissions potentially hazardous to personnel, but the through-web transmission technique requires that the web traverse an open draw in the region of the measuring arrangement. In tissue machines, this is disadvantageous because the tissue web is weak and hence can tend to break in open draws.
The present invention seeks to address the above-noted needs, by providing measuring arrangements that facilitate shortening of the dry end of a tissue machine and that allow paper properties to be measured without the requirement of an open draw. In accordance with the invention, the paper web is measured for basis weight and other parameters while supported on a fabric or other web support. To this end, a reflectance measurement technique is used in which measuring beams (e.g., electromagnetic waves, acoustic waves such as ultrasonic energy, light waves in the visible or invisible spectrum, or the like) are emitted onto the web on the web support and are reflected from the web back to a sensor. Thus, no open draw is required because the sensors that emit and receive the beams are located on only one side of the web.
In some aspects of the invention, traversing sensors are eliminated in favor of fixed sensors that are not subject to the problems associated with traversing sensors. In other aspects of the invention, traversing sensors are used but they are housed in such a way that they are not susceptible to being fouled with dust or other debris, and a measuring frame is not required for supporting the sensors.
Thus, one aspect of the invention provides an active airfoil for a papermaking machine with a fiber optic measuring device integrated into the airfoil. The active airfoil generally comprises a panel defining a web-supporting surface and a plurality of other walls joined to the panel so as to form an internal chamber that is supplied with air under pressure. The airfoil defines one or more air outlets that discharge air from the chamber and along the web-supporting surface to form an air layer that supports a moving paper web traveling along the web-supporting surface. The fiber optic measuring device comprises a plurality of optical fibers having sensing ends. The optical fibers are arranged in the airfoil such that the sensing ends of the fibers face the moving paper web through one or more apertures in the web-supporting panel of the airfoil. The sensing ends of the fibers are spaced apart in the cross-machine direction so that paper properties can be sensed at a plurality of widthwise locations along the web. The optical sensors preferably employ a reflectance measurement technique in which light waves are emitted from ends of some of the optical fibers and reflected waves are received by ends of others of the optical fibers.
In accordance with a preferred embodiment of the invention, the opposite ends of the optical fibers are connected to a sampling device located remote from the airfoil. The sampling device sequentially samples the optical output signals from the optical fibers, and provides samples of the signals to a further device such as a computer, which can determine the web MD and CD profiles therefrom. The sampling device is capable of sampling all of the optical fibers across the entire width of the web much faster than a traversing sensor can be moved across the width, thereby enabling high- and low-frequency MD variations to be detected. The sampling device can be a mechanical device such as a rotating device that is rotated to be coupled sequentially with the ends of the optical fibers arranged about a circular path; alternatively, the sampling device can accomplish the sampling electronically.
In another embodiment of the invention, an active airfoil houses a traversing sensor that performs reflectance measurements by emitting and receiving through one or more openings in the web-supporting wall of the airfoil. The traversing mechanism is shielded by the airfoil from dust and other debris that can foul conventionally mounted traversing sensors.
It is also possible in accordance with the invention to support the web on an active airfoil, passive airfoil, fabric, or other web support and to mount reflectance measurement sensors adjacent the exposed side of the web on the web support.
In one embodiment of the invention, an active airfoil, whether or not it houses sensors, preferably supports the web from the final drying device up to the reel-up in a tissue machine so that there is no open draw, or at most a very short open draw between the airfoil and the reel-up. When the active airfoil is coupled with reflectance sensors mounted either within or adjacent the airfoil, a particularly compact dry end is provided.
More particularly, an apparatus for a dry end of a tissue machine includes a rotatable reel spool onto which the paper web is wound to form a paper roll, and an active airfoil extending from the dryer to the paper roll. The active airfoil in some embodiments has a downstream edge that forms a nip with the paper roll through which the paper web is guided onto the paper roll. In other embodiments, the airfoil does not form a nip with the paper roll, in which case there can be a very short free draw between the downstream edge of the airfoil and the paper roll. Where the active airfoil forms a nip with the paper roll, the active airfoil can be movable relative to the reel spool for controlling the nip load in the nip. Advantageously, the active airfoil can be rotatable about a pivot axis for controlling the nip load. Alternatively or additionally, the active airfoil can include a downstream edge portion that is flexible and bears against the paper roll to form the nip.
As an alternative to supporting the web on an airfoil, the web can be supported on a belt or fabric that extends up to the reel-up. In this case, the reflectance measuring sensors are mounted on a suitable support adjacent the belt or fabric. The web is positioned between the sensors and the belt or fabric.
The invention enables a number of advantages to be achieved over conventional paper machines. The airfoil with integrated fiber optic measuring device requires no traversing measuring head, and hence complicated traversing mechanisms and the vibrations and cleaning problems that are associated with such measuring heads are eliminated. Integration of the measuring device into the active airfoil rather than on a separate measuring frame or the like also saves space and reduces the footprint of the machine. The optical fibers can be routed internally within the active airfoil, and thus will not cause a dust accumulation that could be a fire hazard. Additionally, MD variations are easily detectable by monitoring each sensor at a given widthwise position along the web with a high frequency or even continuously.
When the reflectance measurement sensors are not integrated into an active airfoil, they can be mounted wherever there is a direct line of sight to the paper web supported on a support such as a fabric or airfoil. For example, the sensors can be mounted adjacent a through-air drying fabric on which the web is carried in the drying section, at a location where the paper web is carried on an outward-facing surface of the fabric such that there is a line of sight to the web. The invention thus enables substantial freedom in placement of the sensors.